I. Field of the Invention
The present invention relates to the amplification of high frequency wireless signals. More particularly, the present invention relates to a method of controlling signal phase and amplitude so that the output of multiple amplifiers can be efficiently combined.
II. Description of the Related Art
In the field of wireless transmitters, multiple amplifiers are often connected in parallel, and used to amplify a single signal. A transmitter which uses multiple amplifiers connected in parallel is called a parallel amplifier transmitter, and embodies a parallel amplifier architecture or design. The outputs of the parallel amplifiers in a transmitter are combined before transmission through one or more antennas.
The parallel amplifier architecture allows the use of smaller, less expensive amplifiers. Upon the failure of one of its multiple amplifiers, a parallel amplifier transmitter will not suffer a complete service outage, but will instead exhibit only a decrease in output power. In a single-amplifier design, the failure of a single amplifier will cause a service outage for the entire transmitter. For this reason, a single amplifier in a transmitter may be considered a single point of failure.
Unfortunately, efficient combining of the output of several parallel amplifiers is not trivial. Amplifiers vary in amplitude and phase characteristics such that the same signal fed into several amplifiers will generally result in a slightly different output signal from each amplifier. Unless the output signals of parallel amplifiers are nearly in-phase, they cannot be efficiently combined into the strongest combined output signal. In the worst case, amplifier outputs which are 180 degrees out of phase will destructively interfere with each other, resulting in minimal combined output power.
Several devices for combining multiple amplified signals are known in the art, and include in-phase combiners such as Wilkinson combiners, and quadrature phase combiners, such as Lange couplers. A Wilkinson combiner has two inputs and a single output, with the output generally representing the sum of the input signals. A Lange coupler also has two inputs, one of which is rotated 90 degrees prior to combining. In addition, a Lange coupler outputs a phase difference signal which may be used to determine the phase difference between the two input signals.
In a transmitter that uses multiple parallel amplifiers, each amplifier must typically be tuned at the factory to insure that the phase characteristics of the amplifiers are within some nominal range of each other. To enable such factory tuning, amplifiers are designed with phase trimming circuits such as potentiometers and varactors, both known in the art. Such factory tuning steps must be performed by qualified factory technicians, and are time consuming and costly. It would therefore be desirable to be able to eliminate such factory tuning steps.
Even after tuning amplifiers in the factory, additional measures are required to allow combining of signals from parallel amplifiers. Phase characteristics vary over temperature for each individual amplifier, as well as over time as each amplifier ages. In order to mitigate such amplifier phase variations, methods have been developed to perform real-time phase tuning of parallel amplifiers.
In order to enable real-time phase tuning of parallel amplifiers, some subset of the amplifiers must be equipped with the means to alter the phase of the output. This is typically done by inserting a voltage-controlled phase shifter between the signal source and the amplifier input. The analog control voltage used to control the phase shifter is derived by measuring the signals being provided to a combiner. In a design utilizing a Lange coupler, the Lange coupler""s phase difference signal may be used in a control loop to adjust the control voltage of the phase shifter.
Problems remain with this method of aligning parallel amplifiers. Phase shifters, such as the types using varactors, have non-linear responses which introduce signal distortion into the phase-shifted output. Such distortion may be unacceptable in transmitting a high frequency signal. If the transmit signal is high frequency, then very fine adjustments in phase are necessary to prevent destructive interference. The resolution of a phase shifter may not be fine enough for use in high frequency parallel amplifiers. In addition, the circuits used to produce control voltages for the phase shifter will be subject to variation over time and temperature. Accounting for time and temperature variation further complicates the design of the control loop circuit which provides the phase shifter control voltage.
In addition, there is still a need to perform tuning of amplifiers in the factory, even if only to get the phase output close enough to allow proper functioning of the phase shifter control loop. It might be possible to eliminate the need for factory tuning by using precision components in the construction of the amplifier, but the use of such components would add to the material cost to the amplifier.
In existing designs using in-phase combiners, phase detector circuits are added to measure the phase difference between the inputs to the combiner. The phase detector circuits produce phase difference signal voltages that are provided to control loop circuits which provide analog control voltages to voltage-controlled phase shifters. Any lack of calibration in the phase detector circuits or phase distortion which occurs beyond the phase detector detracts from the combined output of the parallel amplifiers. Because the phase detectors, phase shifters, and control loop circuits are analog, they are subject to changes in characteristics over temperature and age.
In a parallel amplifier architecture which utilizes more than two amplifiers, multiple combiners may be cascaded to form the final combined output signal. At each layer of such a combiner cascade, however, additional phase variation may be introduced which detracts from the effectiveness of phase measurements at the individual amplifier outputs.
A parallel amplifier architecture is desired which efficiently combines the output of multiple parallel amplifiers. In addition, it is desirable that such a design not require expensive, high-precision components and not necessitate factory tuning. Furthermore, it is desirable that such a design be immune to changes in circuit behavior over temperature and over time.
The present invention solves the problems described above by using digital techniques to adjust the phase of source signals as they are generated. In an exemplary embodiment, direct digital synthesizers are used to produce phase-controlled upconverter mixing signals with very fine phase resolution. In another embodiment, digital signal processing techniques are used to perform linear filtering of signals in the digital domain, carefully controlling group delay to produce accurate phase shifting of amplifier input signals. The phase of the input signal provided to each amplifier is adjusted in real-time by a control module, which adjusts amplifier input signals to maximize the power measured at the output of the combiner or combiner network.
Because power measurements are used to optimize the input signal phase of each amplifier, the present invention may utilize either in-phase combiners such as Wilkinson combiners, quadrature phase combiners such as Lange couplers, or other types of signal combiners as appropriate.
Additionally, the output amplitudes of each of the parallel amplifiers are measured and balanced in real time. In addition to prolonging average MTBF of the amplifiers, balancing the outputs of parallel amplifiers having similar performance specifications reduces the chances of overdriving any one of them.
The present invention may be used in any system which allows digital manipulation of the transmit signals used as input to parallel amplifiers.